The present invention relates generally to the field of electrophoretic displays and, specifically, to a novel method of manufacturing such displays.
The electrophoretic display is a non-emissive device based on the electrophoresis phenomenon of charged pigment particles suspended in a solvent. It was first proposed in 1969. The display usually comprises two plates with electrodes placed opposing each other, separated by using spacers. One of the electrodes is usually transparent. A suspension composed of a colored solvent and charged pigment particles is enclosed between the two plates. When a voltage difference is imposed between the two electrodes, the pigment particles migrate to one side and then either the color of the pigment or the color of the solvent can be seen according to the polarity of the voltage difference.
In order to prevent undesired movement of the particles, such as sedimentation, partitions between the two electrodes were proposed for dividing the space into smaller cells. However, in the case of partition-type electrophoretic displays, some difficulties were encountered in the formation of the partitions and the process of enclosing the suspension. Furthermore, it was also difficult to keep different colors of suspensions separate from each other in the partition-type electrophoretic display.
Subsequently, attempts were made to enclose the suspension in microcapsules. U.S. Pat. Nos. 5,961,804 and 5,930,026 describe microencapsulated electrophoretic displays. The reference display has a substantially two dimensional arrangement of microcapsules each having therein an electrophoretic composition of a dielectric fluid and a suspension of charged pigment particles that visually contrast with the dielectric solvent. The microcapsules can be formed by interfacial polymerization, in-situ polymerization or other known methods such as physical processes, in-liquid curing or simple/complex coacervation. The microcapsules, after their formation, may be injected into a cell housing two spaced-apart electrodes, or xe2x80x9cprintedxe2x80x9d into or coated on a transparent conductor film. The microcapsules may also be immobilized within a transparent matrix or binder that is itself sandwiched between the two electrodes.
The electrophoretic displays prepared by these prior art processes, in particular the microencapsulation process as disclosed in U.S. Pat. Nos. 5,930,026, 5,961,804, and 6,017,584, have many shortcomings. For example, the electrophoretic display manufactured by the microencapsulation process suffers from sensitivity to environmental changes (in particular sensitivity to moisture and temperature) due to the wall chemistry of the microcapsules. Secondly the electrophoretic display based on the microcapsules has poor scratch resistance due to the thin wall and large particle size of the microcapsules. To improve the handleability of the display, microcapsules are embedded in a large quantity of polymer matrix which results in a slow response time due to the large distance between the two electrodes and a low contrast ratio due to the low payload of pigment particles. It is also difficult to increase the surface charge density on the pigment particles because charge-controlling agents tend to diffuse to the water/oil interface during the microencapsulation process. The low charge density or zeta potential of the pigment particles in the microcapsules also results in a slow response rate. Furthermore, because of the large particle size and broad size distribution of the microcapsules, the prior art electrophoretic display of this type has poor resolution and addressability for color applications.
To prevent undesired movements of the particles such as lateral migration or sedimentation, partition of the electrophoretic display into smaller cells by photolithographic process has been reported. The process in the prior art is batchwise and requires solvent development. A roll-to-roll microembossing process has also been disclosed. It is desirable to have a high throughput method of manufacture for micro-cups used in electrophoretic or liquid crystal displays that does not require a solvent.
The present invention is directed to a method of manufacture for an array of micro-cups and uses for the micro-cup array.
In one aspect of the invention there is a method utilizing a pre-patterned male mold that is coated with a thermoplastic or thermoset precursor composition (such as, for example, a UV curable resin) to form a micro-cup array. The resin is then contacted with a transfer sheet (or plastic substrate) having a patterned conducting layer and, optionally, heated. The mold is registered to the conductor pattern. A uniform pressure may be applied to the transfer sheet to aid in improving adhesion between the transfer sheet and the resin and control the thickness of the floor of the micro-cups. The resin is cured by exposure to radiation such as UV light. Once cured the resin is released from the male mold to yield the array of micro-cups. Optionally, the male mold may be pre-coated with a release coating such as wax, silicone or fluorinated polymer. If necessary, the micro-cup array may be post-cured.
In a second aspect of the invention there is provided a method of manufacture of an electrophoretic display using the micro-cup array. The process for the manufacture of a full color electrophoretic display comprises laminating the preformed microcups with a layer of positively working photoresist, selectively opening a certain number of the microcups by imagewise exposing the positive photoresist, followed by developing the resist, filling the opened cups with a colored electrophoretic fluid, and sealing the filled microcups by a sealing process. These steps may be repeated to create sealed microcups filled with electrophoretic fluids of different colors.
In a third aspect of the invention there is provided an electrophoretic display using the micro-cup array. The micro-cup array is filled with a dielectric fluid containing at least a charged pigment suspension in a colored dielectric solvent or solvent mixture. The micro-cups are then sealed. The sealed array is laminated with a conductor film pre-coated with an adhesive layer.